Method for treating material



Oct. 12, 1937. B. M. CARTER METHOD FOR TREATING MTERIAL I Filed Jan. 3, 1935 2v Sheets-Sheet l Oct. 12,1937. 'B'. M. CARTER METHOD FOR TREATING MATERIAL.

Filed Jan. 3, 1935 2 Sheets-Sheet 2 57W@ ATTORNEY atente @et l2, l? y METHD FOR TREATING: MTERIAL Application January 3,

7 Claims.

pass the material to be driedthrough an elongated rotary drum against a countercurrent flow of heated drying medium such as air. Because of the low melting point of the salts or similar materials being dried, it is necessary in practical operations that the highest temperature of the drying air be relatively low, i. e., considerably less than the softening point of the material. In countercurrent drying, the nature of the operation is such that the temperature of the heated drying air is greatest at the air inlet-salt outlet end of the drying zone or chamber. The temperature of the heated drying air decreases as o the-air current passes through the drying zone, and is at a minimum at the point where the air leaves the salt inlet-air outlet end of the drying zone.. Hence, the moisture-carrying capacity of n the air current is at a minimum as the air stream leaves the drying chamber. Accordingly, the moisture-carrying capacity of the air as it is discharged from the drier, together with other factors, limits the production of dry material by a single unit of apparatus.

`The invention aims principally to provide a method for drying salts and similar materials by which the output of a unit of given size may be largely increased. A further object of the inven- 4 tion resides in the provision of apparatus for carrying out the improved method.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the 45 apparatus embodying features of construction, combinations of elements and arrangements of parts adapted to eiect such steps. A further understanding of the objects and advantages of the invention may be had from a consideration of the following description taken in connection with the accompanying drawings in which:

Fig. 1 is .a vertical, longitudinal section of one end of the dryer, showing in elevation an axially disposed gas distributing pipe;

1933, Serial No. 649,786

Fig. 2 is a plan view of the apparatus;

Fig. 3 is an enlarged, longitudinal vertical section of the gas distributing pipe;

Fig. 4 is a transverse section on line tof Fig. 3; y

Fig. 5 is a transverse section on the line 5-5 of Fig, 3;

Fig. 6 is a transverse section on line 6-6 of Fig. l;

Fig. 7 is a similar transverse section on line 'l-l of Fig. 1;

Fig. 8 is an enlarged view, partly in section, of that portion of the gas distributing pipe between sections 4-4 and 5-5, Fig. 3

Fig. 9 is a transverse sction on line 9--9 of Fig. 3;

Fig. 10 is a developed plan of a damper in the gas distributing pipe;

Fig. 11 is an end elevation of the damper taken on the line lI--II of Fig. 10;

Fig. 12 shows temperature curves, the purpose of which will be hereinafter noted, and

Fig. 13 is an elevation taken approximately on the line |3'I3 of Fig. 1. I

Referring to Fig. 1 of the drawings, the refer ence numeral 5 indicates one end of a cylindrical drum providing a drying chamber 6. The `drum carriles several longitudinally spaced, circumferential tires 1 resting in supporting rollers 8. The end of the drier, not shown, has mounted thereon an annular gear meshing with a drive pinion through which'4 the drum is rotated from a source of power. The salt outletend 9 of the drum projects into a fixed housing I0 into the bottom of which material is continuously discharged from the drum. One vertical side wall of the housing I0 has attached thereto a circular flange Il engaging a flange l2 carried by the tire-1, anges H and I2 forming a substantially gas-tight joint between the rotary drum and the xed housing Ill.l Material may be continuously withdrawn from the housing by a screw conveyor I3.

The opposite end of the drum, the salt inletair outlet end, not shown in the drawings, extends into a fixed housing similar to housing I0, having an outlet pipe for discharging moisture-laden air from the apparatus. Such fixed housing is also equipped with conventional means for continuously feeding material into the salt inlet end of the rotary drum. As is customary in apparatus of this type, the salt inlet end of the cylinder is elevated slightly to facilitate movement of material gradually toward the salt discharge end in fixed housing Ill. The drum is rotated in the direction of the arrow I5 in Fig. 6.

Reference numeral I6, Figs. 1 and 2, indicates a blower, driven by motor Il, the pressure side of the blower being connected through conduit I8 and opening I9 with the interior of housing I0. In operation, drying medium, such as heated air, is fed into the salt discharge end 9 of the drum from the housing Ill, passes through drying chamber 6 countercurrent to the direction of movement of the salt, and flows from the salt inlet end of the drum into a fixed housing similar to housing Il), from which the air is eventually discharged to the atmosphere.

As shown in Figs. 1 and 7, projecting from the inner face of the cylinder 5 are longitudinal angle irons 20 to which are attached serrated plates 2|, shown in elevation in Fig. 1. In the particular embodiment of the apparatus described, the serrated plates extend longitudinally about half the length of the drum, and, during rotation of the latter, carry material being dried toward the top of the drying chamber 6 and shower the material into the current of hot air passing through the drier in the direction of the arrow 22 (Fig. 1). Mounted in the drum, in staggered relation, near the salt discharge end are baffles or grilles 23, 24 and 25 shown in side and end elevations in Figs. 1 and 7 respectively. The battles are screen-like frames, connected at longitudinal edges 26 and 2l with adjacent serrated plates 29. The'bales prevent lumps from passing thru with dry salt, and effect a more thorough cascading of ne, nearly dried material than is secured by the serrated angles. At these grilles, the nearly dry salt is eiectively showered through the drying atmosphere in a manner which could not be eiected at the opposite-end of the kiln where the salt is wet.

Additional quantities of heated drying medium are fed into the drum through an axially disposed, longitudinal distributing pipe 30 shown in elevation in Fig. 1, and in enlarged section in Fig. 3. The inlet end of pipe 30 vis xedly supported at 3| in outer side wall 32 of the housing I0. Pipe 30 is cylindrical from .wall 32 to section 5 5 lin Fig. 3. From this point, pipe 30 tapers uniformly toward the small end 33, which is closed off by a circular disk 34. vRigidly connected to and extending through disk 34 is a stub shaft 35, the outer end of which passes through a bearing 36 supported by a spider, the arms 31 of which are riveted at the outer ends to angle irons 2|) and at the inner ends to the circular flange 38 of the bearing 36. Hence, the small end of the xed pipe 3|) is maintained in position by bearing 36. The gaseous drying medium, such as air, is fed into pipe 30 from an inlet conduit 40. The top s ide of pipe 30 may advantageously be covered with insulating material, such as asbestos, to prevent overheating and possible melting of salt material falling on top of the pipe during rotation of the drum.

The tapered portion 4| of the air pipe 30, between transverse sections 4 4 and 5 5, is provided on the lower side with a plurality of elongated openings 45, 46, 41, 48, 49, and 5|] of uniform length and width. The openings are separated by the .short sections 5| left uncut to avoid weakening of the pipe. In eiect, all of the openings together may be considered as constituting a single elongated slot in the underside of tapered section 4|. As noted, the drum rotates in the direction of the arrow I5 in Fig. 6, and accordingly the shell, in conjunction with angles 2U and plates 2|, tends to carry material upwardly through the left half 53 (Fig. 6) of drying chamber 6. In order to cause air entering the drying chamber 6 through openings I5-58 Ato be directed toward the salt being dried, the longitudinal center line of openings I5-5U is displaced circumferentially from the vertical a distance equal to an angle A as indicated particularly in Fig. 8, and also in Figs. 4 and 5. Angle A may for example be about 20. The openings are therefore off the vertical center in the direction of rotation of the cylinder so that the hot air is directed toward the surfacey of the bed of salt at a point at which the bed is of substantial depth.

The passage of air through openings 45--58 is controlled by an eccentric damper indicated generally by reference numeral 60, and shown in Fig. 10 in plan. In Fig. 10, the damper is curved upwardly out of the plane of the paper as will be seen from Fig. ll. The curvature of' the damper is such that the convex surface conforms with the inner surface of tapered section 4| of the air pipe 3U. 'I'he damper is rigidly connected through hangers 6I to a longitudinally extending pipe 62, the inner end 63 of which ts over and may oscillate about the inner end of iixed stub shaft 35. The opposite end of pipe 62 is rotatably supported in a xed bearing 65 shown in Figs. l and 3. Keyed to the outer end of pipe 62 is an operating handle 66 (Fig. 13) and since the inner end of the pipe is rotatably carried by stub shaft 35 and the outer end of the pipe is rotatably supported in vbearing 65, the position of the damper 6|) relative to the openings 45-50 may be controlled by manipulation of handle 66.

Referring to Fig. 10, line 68 represents the center line or crest of the damper. When the latter is positioned to close all openings 4550, as shown in the large section Fig. 8, line 68 coincides with the longitudinal center of openings 45--50 indicated by the dotted line between points B and C (Fig. 8). In the particular damper illustrated, although specific dimensions are immaterial, dimension D (Fig. 10) is slightly less than dimension E (measurements being taken from the center line 68 on the convex surface to edges 'I2 and 13). Dimension F is greater than dimension E, and dimension G is somewhaty less than twice dimension F. Thus as shown in Fig. 10, edges 'I2 and 'I3 are disposed in different angular relation with respect to center line 68, that is, the taper of edge I2 with respect to line 68 is sharper than the taper of edge 'I3 with respect to center line 68. When the damper is in place in pipe 30 and adjusted so that all openings 45-50 are closed, longitudinal edges 'I2 and 1,3 and transverse edges represented by dimensions D, F, E and G occupy the positions shown in Fig. 8.

The gas distributing pipe 3|) is also provided at 'I5 (Fig. 1) with a plurality of openings 'I6 shown in section in Fig. 9. Openings 'I6 are to the right of the vertical (Fig. 9) and are arranged similarly to openings 45-50 so that hot gas entering the drum through openings I6 is directed into the body of salt being carried upwardly in the direction of the arrow I5 of Fig. 6. Openings 16 are controlled byla longitudinally sliding damper 'I8 (Fig. 3)y comprising a ring 'I9 carried by spider arms 80, the inner ends of which are connected to a sliding sleeve 8| `extending outwardly through bearing 65 (Fig. l) Attached to the outer end of lsleeve 8| is a lug 84 (Fig. 3)

to which is pivotally connected at 35 -anoperating handle 86 one end of which is pivoted at aooaaaa Tft 01 to the end of a link 88 pivoted to a fixed bracket 89, shown more clearly in Fig. 1. By this construction, it will be seen that movement of the handle 86 from the position shown in full lines in Fig. 3 to the dotted line 90 eifects longitudinal movement of damper ring 'I9 away from openings 16.

The invention may be employed in practice substantially as follows: l

As noted, the invention is directed particularly toga method and apparatus for drying material subject to `softening or liquefaction at relatively low temperatures, and hence should be dried at temperatures below the softening point. For convenience and by way of illustration, the operation of the invention will be described in connection with the drying of trisodium phosphate, a salt containing considerabl quantity of water of cryswhich liquefaction or incipient fusion of the may terial treated would take place.

In customary commercial practice, trisodium phosphate crystals, containing 12 molecules of water, are first separated from the mother liquor by means of a filter, and may be washed with water to remove mother liquor ordinarily retained by the dewatered crystals. For example, after the water wash, one well known -type of centrifugal lter delivers a product containing from to 8% free water. Initial dewatering of the crystal slurry and the apparatus employed constitute no part of this invention.

To meet certain trade requirements on trisodium phosphate crystals, for example, all free water and a small amount of the water of crystallizationmust be removed from the centrifuged salt so that the dried product will contain the equivalent of not less than about 101.5% of NaaPO4.12I-I2O and such water is removed by exposure of the wetl salt to a heated gaseous drying medium such as hot air. Heretofore, drying has been accomplished in operations involving counter-current flow of salt and hot air through a rotary drum, the hot air initially introduced into `the drier at the salt discharge end being heated to the desired temperatures by any convenient-means. In such operations, involving countercurrent flow of salt and drying air, the

amount of salt which may be dried in any given apparatus is dependent upon the difference in water content between the air entering the salt outlet end and leaving the salt inlet end of the drier. Furthermore, the output of such a drier per unit of time is more or' less dependent upon atmosphericconditions, the capacity being less in summer than in Winter on account of the higher temperature and the humidity of the atmosphere in warm weather.

The basic features of the invention may be more clearly understood from a brief consideration of some of the fundamental principles involved. In

heating to, say. 120 F., such air, as introduced into the salt outlet end of the drier, would contain about 7.1 grains of water per ,cubic foot and the relative humidity decreases to about 21%. Since in practical operation, complete equilibrium is not obtained, the humidity of the heated air discharged from the salt inlet end of the drier decreases as the temperature increases. Thus, air leaving the salt inlet end of the drier at about 100 F. and relative humidity of about 60% 10 would carry about 11.6 grains of waterper cubic foot, or about 4.2 grains per cubic foot in excess of entering atmospheric air heated to about 100 F., having a relative humidity of about 38% and` containing about 7.4 grains of water per cubic 15 foot. Further, air leaving the salt inlet end of the drier at about 120 F. and a relative humidity of about 50% would carry about 17.2 grains of water per cubic foot, or about 10.1 grains per cubic foot inexcess of entering atmospheric air heated to about 120 F., having a relative humidity of about 21% and containing about '7.1 grains of water per cubic foot. Accordingly, a given volume of air leaving the drier at about 120 F. and about 50% relative humidity would dry more 25 than twice the amount of wet salt that would be dried by the same amount of air leaving the drier at about 100 F. and about 60% relative humidity.

It will be understood the output per unit of time of a drier of a given size is determined by quantity of air employed, moisture content of such air, and the temperature to which the air is heated prior to introduction to the drier.' The initial moisture content of the air is determined by atmospheric conditions, and is not subject to control, unless the air is previously conditioned by refrigeration, drying, etc. The remaining two controllable factors are also subject to limitation. The volume of drying air employed cannot be in 40 excess of that at which it would tend to carry the be exposed to temperatures in excess of about 140 F. without creating a sticky condition that produces undesirable caking. Thus, in practice it is not desirable to allow the maximum temperature of the drying air to exceed about 135 F. In prior practice, it has been customary to pass the salt continuously through a rotary drum in contact with a stream of air flowing counter-current through the drier, the temperature of the drying air introduced into the salt outlet end of the drier being about 135 F.

It has been found that in prior practice, e. g. Where wet salt and drying air pass countercurrent through the drier, the temperature of the drying air drops off rapidly, in the rst one-third of the length of the drying chamber, and thereafter-re-V mains substantially constant until the air leaves the salt inlet end of the drier. In Fig. 12, the abscissa represents the length of the drying chamber 6, and the ordinate represents the temperature of the drying air.. Dotted curveK approximately indicates the temperature conditions existing in the drier when operated in accordance with prior practice. From this curve, it will be seen that while passing through approximately the rst third of the drier, the temperature drops from approximately 135F. to, say, about 11o-112 F., and thereafter, during the subsequent two-thirds of passage through the drier, the temperature of the drying air decreases gradually to about 100 F., at which temperature the drying air passes out of contact with the wet salt being fed into the salt inlet end of the drier.

In accordance with the present invention, operations are conducted so that the temperature of `the drying air leaving the drier is considerably raised, thus largely increasing the water carrying capacity of the drying air at the time of last contact with wet salt. By means of the present method and apparatus, conditions in the drying chamber may be so controlled that the temperature of the air leaving the drier may be regulated as desired within certain limits. From a consideration of thetemperature curve K of Fig. 12, it will be seen that if properly regulated quantities of drying gas, heated to proper temperatures, are introduced into the drier beginning at points beyond approximately the rst third of the drier (measuring from the salt outlet end), the temperature of the drying atmosphere in the subsequent two-thirds of the drying chamber may be raised, and the water carrying capacity of the drying air, as discharged from the salt inlet end of the drier, largely increased. In the present inventon, such temperature control in the drier may be had by introducing heated gas into the drying chamber 6 through the gas distributing pipe 3U, by suitable regulation of dampers '60 and 18.

In the apparatus illustrated, it may be assumed for example, drum 5 is about 30 feet long. The taper of pipe 30 begins at section 5 5, Fig. 3, about 7 feet from the salt outlet end 9 of the drum, and at a point which is indicated by a corresponding line 5 5 on Fig. 12. As the temperature of the drying air has dropped materially at about line 5 5 (Fig. 12), to accomplish the purposes of the invention, controlled quantities of gas heated to proper temperatures are introduced into the drying chamber through one or more of the openings 45 50 of pipe 30. It is noted, section line 4 4 (Fig. 3) corresponds approximately with line 4 4 on Fig, 12. The particular portion of the drier, between lines 5 5 and 4 4 (Fig. 12) into which hot gas from pipe 30 is introduced is selected by manipulation of the damper 60.

In Fig. 8, the damper is shown in such position that all openings 45-50 are closed. It will be recalled from the foregoing description that edges 'l2 and 13 are pitched oppositely and at different angles with respect to center line 58 of damper 69.

Referringto Fig. 8, it will be seen that movement of damper 60 in the direction of the arrow 95, first uncovers a portion or the whole of opening 45 nearest the small end of tapered section lll. Thus, a comparatively short downward movement of handle 66 (Fig. 13) uncovers the opening 45, and admits heated gas from the pipe 30 into the interior of the drum at the point at which opening 45 is positioned. Because of the relatively sharp pitch of edge 12, with respect to center line '68 of the damper 60, it will be seen that continued movement of damper 60 in the direction of the arrow 95 successively uncovers openings 45-50 in the order named, until when the handle 66 is in the position of the dotted line 96 (Fig. 13), all openings I5-L50 are uncovered, and because of the taper of pipe 30 substantially equal quantities of air pass through each opening into the drum. Thus, when it is desired to introduce air into parts of chamber 6 nearer the salt inlet end than salt outlet end 9, handle '66 is moved downwardly, and as many of the openings as necessary may be uncovered, and additional hot gas admitted to selected portions of the drum.

In other situations arising in practice, it may be desirable to introduce additional hot gas only or intially in portions of the drying chamber nearer the salt discharge end of the drum. This may be accomplished by raising handle 66 which causes movement of damper 60 in the direction of the arrow 91 in Fig. 8. As edge E of the damper is shorter than edge F, it will be seen that such movement of the damper rst uncovers the opening 50 adjacent the section line 5 5 of Fig. 3. On account ofthe pitch of edge 13, with respect to center line 68 of damper 6U, further movement of the damper in the direction of the arrow 91 uncovers openings 50 45 progressively in the order named. When handle 66 is in the position of dotted line 99, all openings are uncovered, and airenters drying chamber 6 in the same manner as noted above where the damper was fully opened by downward movement of handle 66.

Thus, by moving handle 66 downwardly, air is introduced into the drier rst at the small end of the tapered section, and then progressively toward the larger section, if downward movement of handle 66 is continued. On the other hand, by lifting handle 66, air is introduced into the drum first through the opening 5l) adjacent section line 5 5 of Fig. 3, and thereafter, on further upward movement of handle 66, through openings progressively nearer the small end of a distributing pipe 30. Accordingly, by adjustment of the position of the damper to suit specic operating conditions, additional quantities of hot drying gas may be introduced into any portion of the drying chamber, and the temperature conditions therein controlled as required. It will be seen that by reason of the difference of pitch of edges 12 and 'i3 with respect to center line 68 of the damper, all the openings 50 45 may be more rapidly opened or closed by raising handle 66 or lowering the same to the horizontal position, thus in eiect using edge 'i3 as the working edge of the damper. On the other hand, downward movement of handle 66 and raising the same to the horizontal position makes edge 'l2 the working edge, and as the pitch of edge 'l2 is sharper, with respect to center line 68, the openings 45 5D are uncovered or covered at a less rapid rate. In practice, it has been found generally desirable to admit additional hot drying gas nearer the salt inlet end, say through openings 45, 46 and 4l, rather than near the salt outlet end. Thus, handle 66 is manipulated so that edge 12 is employed as the working edge of the damper.

In one particular series of tests, carried out in midsummer over an extended period of time, and at an average atmospheric temperature of about 80 F. and about 70% relative humidity, operations were conducted so that approximately 40 tons of dried trisodium phosphate were produced by the drier in 24 hours, about 10.8% free and combined water having been removed from the wet salt. The total quantity of drying medium employed was about 5000 cubic feet per minute. About half this volume of air, at temperatures about 131 F., was fed into the drier from the fixed housing l0. An equal volume, consisting of products of combustion from an oil burner, heated to temperatures of about 400 F. was fed into the distributing pipe 30 from pipe il), and intro- Feed Product Percejnt Percent NB3PO4.12H20 92. 96 103. 04 NaOH l. 60 1. 25 NazC On 7SA 1.23

By proceeding in accordance with'the above,'the output of the drier was approximately doubled over operations carried out in a drier of the same size, but built and operated as in the prior practice. It will be seen from Fig. 12, the temperature of the drying medium leaving the drier is about 20 higher than previously obtainable, thus effecting the largely increased capacity of the drier.

By operation ofY damper 18 controlling openings 16 positioned relatively near the salt discharge end of the drum, additional quantities of hot drying gas may be fed into the drum if desired, to regulate temperatures in the drying chamber, and to provide for particular operating conditions arising. The construction including damper 1B also permits operation ofthe process in such manner that the air fed into the drier from the housing I0 may be at vtemperatures of about 100 F., thus cooling the dried salts, to some extent, just prior to discharge from the drier, such cooling advantageously permitting packaging of the product without caking, and Without the use of additional equipment to cool the product prior to packaging.

'Where air or other gaseous heating medium is introduced into the drying chamber from housing I 0 at lower temperatures of about, say, 100 F., the temperature in the drying chamber just beyond the salt outlet end 9 may be rapidly raised to the desired maximum temperature by opening the damper 18, and permitting admission of hot gases through openings 16. By this operation in conjunction with manipulation of damper 68, a temperature curve in the drier approximating 'curve M may be obtained, which curve, as will be seen, converges with curve L at a point near the salt outlet end of the drier, the ternperatures thereafter approximating those which prevail in instances where the drying air is initially introduced at 131-132 F.

In the above specific example, it will be noted the hot combustion products entering the drier through pipe 30 were heated to temperatures of about 400 F. No particular temperatures for the high .temperature gas supply need be employed. Improvedresults, over prior practice, may be obtained Where the hot gases entering through pipe 30 are at any temperature above that of the gas entering the drier from housing I0. The higher the temperature of the gases entering'the drier from pipe 30, the higher the temperature of the combined gas stream leaving the salt inlet end of the drier, and accordingly the greater the output of the drier and the eiiiciency of the dryingoperation. To obtain substantially improvedincrease in the drier capacity, the tcmperature of the gas introduced throughpipe 30 should not be less than about, say, 250 F. The

upper limit of the temperature of the gas from pipe 30 is controlled `by the nature of the material being treated. The temperature of such gas should not be high enough to raise the temperature of the combined gas stream in the drier sufficient to cause softening or injury to the material being treated. The v olume of the combined gas stream in the drier should not be in excess of that at which material would be carried away in suspension.

It will also be understood any suitable gas may be used for both the high and low temperature supply. Air heated by heat exchange may be employed for either or both low and high temperature supply, and also combustion gases, if sootless, may be utilized for either or both the low and high temperature Supply, thus eliminating use of steam heaters previously employed, and thereby effecting economies in cost of heat supplied to the drier.

I claim:

1. The method of drying solid material susceptible to softening at elevated temperatures which comprises introducing into one end of a drying zone containing a body of the material a primary stream of gaseous drying medium having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening of the solid material, passing the primary stream of drying medium through the drying zone in contact with the solid material therein, adding to and mixing with the primary stream during passage thereof through the zone secondary quantities of gaseous drying medium heated to temperatures substantially in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition confines of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone and at points substantially spaced from the outer confines of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at tem-perature approaching but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium with the material and to prevent heating. of the material by the resulting combined gas stream to temperatures suiiicient to soften the material.

2- The method of drying solid material susceptible to softening at elevated temperatures which Ycomprises introducing into one end of a drying zone containing a body of the material a primary stream of gaseous drying medium having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening of the solid material, passing the primary'stream of drying medium through the drying zone in contact with the solid material therein, adding to and mixing with the primary stream during passage thereof through the zone secondary quantities of gaseous drying medium heated to temperatures substantially in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in msulated condition confines of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone through said heat insulated confines of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature approaching but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium with the material and to prevent heating of the material by the resulting combined gas stream to temperatures sufficient to soften the material.

3. The method of drying solid material susceptible to softening at elevated temperatures which comprises introducing into one end of a Arelatively elongated drying zone containing a body of the material a primary stream of gaseous drying medium having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening .of the solid material, passing the primary stream of drying medium through the drying zone in contact with the solid material therein, adding to andmixing with a substantial length of the primary stream during passage thereof through the zone, secondary quantities of gaseous drying medium heated to temperatures substantially in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition connes of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone and at points substantially spaced from the outer confines of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature approaching but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium with the material and to prevent heating of the material by the resulting combined gas stream to temperatures sucient to soften the material.

4. The method of drying solid material susceptible to softening at elevated temperatures which comprises passing countercurrent through a relatively elongated drying zone a body of material to be dried and a primary stream of gaseous drying medium having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening of the solid material, passing the primary stream of drying medium through the drying zone in contact with the solid material therein, adding to and mixing with a substantial length of the primary stream during passage thereof through the zone secondary quantities of gaseous drying medium heated to temperatures substantially'in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition confines of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone and at points substantially spaced from the outer confines of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature approaching but not in excess of the softening point of the material, whereby to prevent any substantiall ceptible to softening at elevated temperatures l which comprises introducing into one end of a relatively elongated drying zone containing a body of the material a primary stream of gaseous drying medium having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening of the solid material, passing the i primary stream of drying medium through the drying zone in contact with the solid material therein, adding to and mixing with the primary stream during passage thereof through the zone secondary quantities of gaseous drying medium heated to temperatures substantially in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition confines of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced substantially axially of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature approaching but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium with the material and to prevent heating of the material by the resulting combined gas stream to temperatures suicient to soften the material.

6. The method of drying solid material susceptible to softening at elevated temperatures which comprises introducing into one end of a relatively elongated drying zone containing a body of the material a primary stream of combustion gases having an initial elevated temperature, at the point of entry into the drying zone, approaching but a little less than that causing softening of the solid material, passing the primary stream of combustion gases through the drying zone in contact with the solid material therein, adding to and mixing with a substantial length of the primary stream during passage thereof through the zone secondary quantities of combustion gases heated to temperatures substantially in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition confines of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone and at points substantially spaced from the outer confines of the drying zone, and controlling the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature approaching but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium withlthe material and to prevent heating of the material by the resulting combined gas stream to temperatures sufficient to soften the material.

'7. The method of drying solid material susceptible to softening at elevated temperatures which comprises introducing into one end of a drying zone containing a body of ythe material a primary stream of gaseous drying medium having an'initial elevated temperature, at the point of entry into the drying zone, approaching but a 4little less than that causing softening o the solid material, passing the primary stream of drying medium through the drying zone in contact with the solid material therein, adding to and mixing with the primary stream during passage thereof through the zone secondary quantities of gaseous drying medium heated to temperatures not less than the orderA of 120 F. in excess of the initial temperature of the primary gas stream and substantially in excess of the temperature causing softening of the material, maintaining in insulated condition connes of the drying zone heated to temperatures in excess of the softening point of the material, said secondary gas being introduced interiorly of the zone and at points substantially spaced from the outer confines of the drying zone, and controlling, the introduction and temperature of such secondary quantities of hot drying medium so as to maintain the resulting stream of mixed drying gases at temperature'approachin'g but not in excess of the softening point of the material, whereby to prevent any substantial direct contact of said hot secondary drying medium with the material and to prevent heating of the material by the resulting combined gas stream to temperatures sufcient to soften the material.

BERNARD M. CARTER. 

